(NBC NEWS) -- A deadly wintry storm system is threatening plans for millions of Americans who will fly or drive somewhere for Thanksgiving - with some of the worst weather expected on Wednesday, the busiest travel day of the year.
Storms have already heaped up to a foot of snow on the mountains of Utah and Colorado and claimed 13 lives, including a 4-year-old girl who was killed in a rollover crash on an icy road in New Mexico.
Now the weather pattern is picking up speed and heading for the Northeast, and the 43 million Americans who plan to travel for Thanksgiving are at risk. Rain and ice sweeping across the South will converge with a storm system pushing down from the Great Lakes.
"That is a lethal combination for winter weather in the Northeast," said Tom Niziol, a winter weather expert for The Weather Channel.
Among the 43 million travelers are 3.1 million expected to fly. Flights could be delayed in New York, Boston, Washington and Baltimore. That is because of expected low clouds and strong wind, said Kevin Roth, lead meteorologist for The Weather Channel.
In the Northeast, the worst weather is expected Tuesday and Wednesday.
"If people traveling can get out before then, or wait until afterward, that would be the best thing," Roth said.
On Monday alone, nearly 600 flights were canceled and scores more delayed across the country, most due to the weather, according to FlightAware.com, a website that tracks airline traffic.
As the storm moved precariously east, the website unveiled its "misery map." Chicago's O'Hare International Airport as well as the airports in Dallas Fort-Worth, New York, Denver and Washington, D.C., showed the most red for misery on the map Monday evening.
The system moving across the South is expected to bring ice to Tennessee, Kentucky, Virginia, West Virginia and parts of the Carolinas late Monday night and into Tuesday.
On Tuesday, it will link up with the system pushing down from the Great Lakes, bringing heavy snow to parts of Pennsylvania and upstate New York, and heavy rain closer to the Northeast coast.
The storm is forecast to dump up to a foot of snow on Buffalo and Syracuse, N.Y., and up to eight inches on Pittsburgh, Roth said.
The massive storm system - which the National Weather Service deemed "complicated" - was not only forecast to dump snow, but sleet and freezing rain were possible hazards from the central Appalachians to northern New England. Rain was almost assured in most parts of the eastern seaboard.
A swath of heavy rain, some 2 to 3 inches of it, will deluge central Gulf Coast up to New England through Wednesday, Dr. Greg Forbes, The Weather Channel's severe weather expert, reported. Some areas may get as many as 4 inches of rain or more with the potential for localized flooding.
After a frigid, blustery weekend in the Northeast, 1,000 people were already without power in Connecticut after high wind brought down a power line, NBC Connecticut reported.
The storm system threatening Thanksgiving plans started in the West on Thursday, where it killed three people in California. It moved on to New Mexico and Texas, both of which were under winter storm warnings Monday.
Up to 11,000 customers were still without power in parts of Texas Monday in the aftermath of outages that peaked at more than 27,000 customers, NBC Dallas Fort-Worth reported. Over the weekend, more than 300 flights in North Texas were cancelled in anticipation of ice.
The storm sped up and moved on through Texas, Oklahoma and Arkansas. Three of the people died in a pileup of a dozen vehicles in the Texas Panhandle, and four were killed in Oklahoma, officials told NBC News. Another man was killed in flooding in Arizona.
A 14-year-old Detroit middle school student, Malik Shelton, was killed after he touched an electrical power line downed by the storm system last week, the medical examiner confirmed Monday. He was walking home from Burns Middle School, closed because of a power outage, when the incident occurred, police said. He was revived and taken to the hospital but died on Saturday. A funeral was planned for Tuesday, NBC station WDIV reported.
By Thanksgiving Day, the system will have moved out over the Atlantic Ocean.
Tom Costello and Jeff Black of NBC News also contributed to this report.
By Alexander Smith and Erin McClam, NBC News